The phase behavior of fats is mainly determined using DSC. Here, the application of temperature modulated optical refractometry (TMOR) is examined to monitor the phase transitions of palm oil with different degrees of saturation. Studying the phase behavior by both methods revealed systematic differences. At identical scan rates, TMOR yielded up to 2 °C higher crystallization temperatures and identified consistently lower temperatures for melting phenomena. Because the prism serves as heating surface and defines the sample volume considered for the measurement a more direct heat transfer with TMOR is assumed. The sample depth above the prism relevant for the determination is only one micron. Hence, a direct heat transfer is ensured and thermal lag is practically eliminated causing the above‐mentioned differences. Because the TMOR signal is averaged over a defined prism surface area data for inhomogeneous samples can be generated. Although actual values for thermal expansion coefficients appear meaningless the combination of the TMOR signals allows to accurately determine the relevant phase transitions. The identification of different polymorphic forms and levels of solids in palm oil will be studied prospectively building on the promising results reported to identify if TMOR can become a valuable extension of the fat technologists' toolbox. Practical Applications: The new temperature modulated optical refractometry can extend the mainly used differential scanning calorimetry. It works highly accurate at small scan rates (<5 °C min−1) in comparison to the DSC. The new method can provide a deeper insight into samples during heating and cooling due to additional temperature undulation as well as the possibility to perform quasi‐isothermal measurements. It is possible to investigate the crystallization and melting behavior of fats with the new temperature modulated optical refractometry (TMOR). An undulated temperature is applied and the time‐delayed answer of the refractive index is measured. Subsequently, the real and imaginary part of the thermal expansion coefficient α are calculated based on the refractive index of the sample and the occurring phase shift. A plot of the real and imaginary part against the temperature leads to peaks at the phase transition temperatures and a resulting thermogram for the sample.
The solid fat content (SFC) at different temperatures is an important characteristic of fat phases because it correlates to functionality in product applications. Consequently, this characteristic is also used to specify fat compositions in trade. Of three methods applicable, pulsed nuclear magnetic resonance (pMNR) is predominantly applied. Dilatometry and differential scanning calorimetry (DSC) find much less application. Handling with glass vials and high equipment costs make the search for alternatives to pNMR a useful endeavor. Optical refractometry is evaluated with respect to its potential to determine SFC values. Since refractometry is in the first place not suited for suspensions the positive results found are surprising. Applying temperature modulated optical refractometry (TMOR), isothermal optical refractometry with a superimposed temperature undulation yields repeatable results that are highly comparable to pNMR data. For the system studied (palm oil, coconut oil, partially hydrogenated palm oil), TMOR clearly outperforms DSC when pNMR is considered the method of reference. The key finding that refractive index is suitable to determine properties of suspensions is accompanied by the indications that refractometry has the potential to enable competitive methods within the fat technology. Practical Applications: The observation that refractometry can deliver quantitative data on fat suspensions enables the development of an array of new analytical methods. Next to SFC values and melting point, studies on the characterization of polymorphism can be envisioned. Since the device is robust and affordable, it could be in product development and quality control.
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